This invention relates to electrical amplifiers, and, more particularly, to a linear current amplifier for amplifying a square wave signal to drive a nonlinear load having a saturable amplifier stage.
Amplifiers for driving a load, such as an electric motor, frequently employ transistor amplifying stages driven between states of saturation or current conduction and cut-off or current nonconduction. The power applied to the load is sufficiently high such that, if a comparable amount of power were dissipated in the transistor amplifier stage immediately preceding the load, the transistor amplifier stage would overheat and fail. During a state of saturation and during a state of nonconduction, minimal power is dissipated in a transistor. Thereby, amplifiers employing transistors operating in the switch mode, switching between states of saturation and nonconduction, can apply the high power to the load without the dissipation of excess heat in the transistor.
The resulting signal applied to the load, thus, has the waveform of a square wave or digitally formatted signal. The amplitudes of the pulses of the succession of pulses of the signal are all equal. Variation of the power applied to the load is accomplished by pulse-width modulation of the square-wave signal to provide a duty cycle commensurate with the desired power which is to be applied to the load. A system for applying power to a load typically comprises in addition to the amplifier, an oscillator generating a square-wave signal, a source of an analog signal having an amplitude designating the requisite duty cycle, and a pulsewidth modulator which is responsive to the analog signal for modulating the duration of the pulses of the square wave signal to provide the requisite duty cycle. The pulse repetition frequency of the signal applied to the load is much higher than the frequency response of the load circuits, namely, the cut-off frequency of the inductive circuit of a motor or the cut-off frequency of the capacitive circuit in a capacitive type load. Thus, the load filters the pulse-train signal to extract a current or voltage equal to the average value thereof. Since the average value is proportional to the duty cycle, the load receives the desired power.
To conserve power in the entire amplifier, other stages, in addition to the stage immediately preceding the load, have been constructed of transistor circuits wherein the transistor is switched between states of saturation and nonconduction. Thus, such amplifiers have been constructed of serially connected transistors operating in the switch mode. Such circuitry has been advantageous in reducing the power dissipation requirements of the transistors and, thereby, permits the use of more economical lower-power transistors.
However, a number of irregularities arise in the use of a series of switch-mode transistor circuits for driving the switch-mode transistor immediately preceding the load, as well as for driving other nonlinear circuits.
A problem arises from the use of a series of switch-mode transistors with current limiting resistors to provide driving current to the transistor amplifying stage immediately preceding the load. A switched-mode transistor amplifying stage, as an example of a saturable driver immediately preceding the load, requires a positive base current to flow in order to induce and sustain saturation. The magnitude of this positive base current must be a dictated percentage of the desired collector current in the saturable transistor driver stage in order to insure efficient switched-mode operation. The simple use of current limiting resistors precludes the possibility of accurate control of the saturating driver transistor with respect to changing driver conditions, wherein these conditions include, but are not limited to, changes in driver temperature, changes in collector current, voltage fluctuations and emmiter ballast resistor voltage drops under parallel driver operation.
A problem arises from the failure of a series of switch-mode transistor circuits to preserve the rise time and fall time, respectively, at the leading and trailing edges of each pulse of the modulated square-wave signal. As a result, the rise and fall times become progressively longer during each amplifying stage, and the square-wave signal is thus modified to a trapezoidal signal. Thus the amplifying stage preceding the load experiences periods of time when the transistor therein is neither in a state of saturation or a state of non-conduction. Excessive heating results, defeating the purpose of the switch-mode operation. Attempts to remedy the situation by use of more rapidly responding transistors which can better preserve the rise and fall times may well be impractical because of their relatively high cost.